Charge control of micro-electromechanical device

ABSTRACT

A charge control circuit for controlling a micro-electromechanical system (MEMS) device having variable capacitor formed by first conductive plate and a second conductive plate separated by a variable gap distance. The charge control circuit comprises a switch circuit configured to receive a reference voltage having a selected voltage level and configured to respond to an enable signal having a duration at least as long as an electrical time constant constant of the MEMS device, but shorter than a mechanical time constant of the MEMS device, to apply the selected voltage level across the first and second plates for the duration to thereby cause a stored charge having a desired magnitude to accumulate on the variable capacitor, wherein the variable gap distance is a function of the magnitude of the stored charge.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is related to U.S. patent application Ser. No.“unassigned” (Attorney Docket No. 10016895-1) filed concurrentlyherewith and entitled “Optical Interference Display Device,” which isherein incorporated by reference.

THE FIELD OF THE INVENTION

The present invention relates to the field of micro-electromechanicaldevices. More particularly, the present invention relates to chargecontrol of a micro-electromechanical device.

BACKGROUND OF THE INVENTION

Micro-electromechanical systems (MEMS) are systems which are developedusing thin film technology and which include both electrical andmicro-mechanical components. MEMS devices are used in a variety ofapplications such as optical display systems, pressure sensors, flowsensors and charge control actuators. MEMS devices use electrostaticforce or energy to move or monitor the movement of micro-mechanicalelectrodes which can store charge. In one type of MEMS device, toachieve a desired result, a gap distance between the electrodes iscontrolled by balancing an electrostatic force and a mechanicalrestoring force. Digital MEMS devices use two gap distances, whileanalog MEMS devices use multiple gap distances.

MEMS devices have been developed using a variety of approaches. In oneapproach, a deformable deflective membrane is positioned over anelectrode and is electrostatically attracted to the electrode. Otherapproaches use flaps or beams of silicon or aluminum which form a topconducting layer. With optical applications, the conducting layer isreflective and is deformed using electrostatic force to scatter lightwhich is incident upon the conducting layer.

One approach for controlling the gap distance is to apply a continuouscontrol voltage to the electrodes, wherein the control voltage isincreased to decrease the gap distance, and vice-versa. However, thisapproach suffers from electrostatic instability that greatly reduces auseable operating range over which the gap distance can be effectivelycontrolled. This is because the electrodes form a variable capacitorwhose capacitance increases as the gap distance decreases. When the gapdistance is reduced to a certain threshold value, usually abouttwo-thirds of an initial gap distance, the electrostatic force ofattraction between the electrodes overcomes the mechanical restoringforce causing the electrodes to “snap” together or to mechanical stops.This is because at a distance less than the minimum threshold value, thecapacitance is increased to a point where excess charge is drawn ontothe electrodes resulting in increased electrostatic attraction—aphenomenon known as “charge runaway.”

This non-linear relationship between the control voltage and the gapdistance limits the controllable range of electrode movement to onlyabout one-third of the initial gap distance, and thus limits thepotential utility of the MEMS device. For example, with optical displaysystems, interference or detraction based light modulator MEMS devicespreferably should have a large range of gap distance control in order tocontrol a greater optical range of visible light scattered by theoptical MEMS device.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a charge control circuitfor controlling a micro-electromechanical system (MEMS) device havingvariable capacitor formed by a first conductive plate and a secondconductive plate separated by a variable gap distance. The chargecontrol circuit comprises a switch circuit configured to receive areference voltage having a selected voltage level and configured torespond to an enable signal having a duration at least as long as anelectrical time constant constant of the MEMS device, but shorter than amechanical time constant of the MEMS device, to apply the selectedvoltage level across the first and second plates for the duration tothereby cause a stored charge having a desired magnitude to accumulateon the variable capacitor, wherein the variable gap distance is afunction of the magnitude of the stored charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary embodiment of amicro-electromechanical system according to the present invention.

FIG. 2 is a diagram illustrating an exemplary embodiment of amicro-electromechanical device.

FIG. 3 is a schematic diagram illustrating an exemplary embodiment of acharge control circuit.

FIG. 4 is a diagram illustrating an exemplary embodiment of amicro-electromechanical system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural or logical changes may bemade without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

FIG. 1 is a diagram illustrating an exemplary embodiment of amicro-electromechanical system 30 according to the present invention.The micro-electromechanical system 30 includes a charge control circuit32 and a micro-electromechanical device 34. Charge control circuit 32further includes a variable power supply 36, a controller 38, and aswitch circuit 40. In the exemplary embodiment, micro-electromechanicaldevice 34 further includes a first conductive plate 42 and a secondconductive plate 44 that form a variable capacitor 46 having a variablegap distance 48, wherein variable gap distance 48 is a function of amagnitude of a stored charge on variable capacitor 46. In oneembodiment, first conductive plate 42 is moveable, while secondconductive plate 44 is fixed to a substrate 50.

Charge control circuit 32 is configured to controlmicro-electromechanical device 34 by applying a reference voltage havinga selected voltage level provided by variable power supply 36 acrossfirst and second conductive plates 42 and 44 for a predeterminedduration to thereby cause a stored charge having a desired magnitude toaccumulate on variable capacitor 46. By application of a precisionreference voltage across first and second conductive plates 42 and 44,the charge stored on variable capacitor 46 and thus, variable gap 48,can be controlled over a wide gap distance range.

In the exemplary embodiment, variable power supply 36 is a variablevoltage source configured to receive a voltage select signal fromcontroller 38 via a path 52 and to provide the reference voltage havinga selected voltage level based on the voltage select signal to switchcircuit 40 via a path 54. Switch circuit 40 is configured to receive anenable signal having a duration from controller 38 via a path 56 and, inresponse, to apply for the duration the selected voltage level tomicro-electromechanical device 34 via a path 60 to thereby cause astored charge having a desired magnitude to accumulate on variablecapacitor 46. In one embodiment, switch circuit 40 is configured toreceive a clear signal from controller 38 via a path 58 and, inresponse, configured to remove a potential stored charge on variablecapacitor 46 to thereby place variable capacitor 46 at a known chargelevel prior to applying the reference voltage having the selectedvoltage level.

FIG. 2 is a diagram illustrating an exemplary embodiment of amicro-electromechanical device 70. In the exemplary embodiment,micro-electromechanical device 70 displays, at least partially, a pixelof a displayable image. The device 70 includes a top reflector 72 and abottom reflector 74, as well as a flexure 80 and a spring mechanism 82.A resonant optical cavity 76 is defined by the reflectors 72 and 74,which has a variable gap distance, or gap distance, 78. The topreflector 72 is in one embodiment semi-transparent or semi-reflective.The bottom reflector 74 is in one embodiment highly reflective orcompletely reflective. In other embodiments, the top reflector 72 ishighly reflective or completely reflective and the bottom reflector 74is semi-transparent or semi-reflective. In various embodiments, springmechanism 82 can be any suitable flexible material, such as a polymer,that has linear or non-linear spring functionality.

In the exemplary embodiment, the optical cavity 76 is variably selectiveof a visible wavelength at an intensity by optical interference.Depending on the desired configuration of micro-electromechanical device70, the optical cavity 76 can either reflect or transmit the wavelengthat the intensity. That is, the cavity 76 can be reflective ortransmissive in nature. No light is generated by optical cavity 76, sothat the device 70 relies on ambient light or light provided bymicro-electromechanical device 70 that is reflected or transmitted bythe cavity 76. The visible wavelength selected by the optical cavity 76,and its intensity selected by the optical cavity 76, are dependent onthe gap distance 78 of the cavity 76. That is, the optical cavity 76 canbe tuned to a desired wavelength at a desired intensity by controllingits gap distance 78.

In the exemplary embodiment, the flexure 80 and the spring mechanism 82allow the gap distance 78 of the optical cavity 76 to vary when anappropriate amount of charge has been stored on the reflectors 72 and74, such that a desired wavelength at a desired intensity is selected.This charge, and the corresponding voltage, is determined in accordancewith the following Equation I, which is the force of attraction betweenthe reflectors 72 and 74 acting as plates of a parallel plate capacitor,and does not take into account fringing fields: Equation  I:${F = \frac{ɛ_{0}V^{2}A}{2d^{2}}},$

where ε₀ is the permittivity of free space;

-   -   V is the voltage across the reflectors 72 and 74;    -   A is the area of each of the reflectors 72 and 74; and    -   d is the gap distance 78.        Thus, a one volt potential across a 70 micron square pixel, with        a gap distance 78 of 0.25 microns, yields an electrostatic force        of 7×10⁻⁷ Newtons (N).

Therefore, an amount of charge corresponding to a small voltage betweenthe reflectors 72 and 74 provides sufficient force to move the topreflector 72, and hold it against gravity and shocks. The electrostaticcharge stored in the reflectors 72 and 74, is sufficient to hold the topreflector 72 in place without additional power. In various embodiments,charge leakage may require occasional refreshing of the charge.

In the exemplary embodiment, the force defined in Equation I is balancedwith the linear spring force provided by the spring mechanism 82according to the following Equation II:

Equation II:F=k(d ₀ −d),

where k is the linear spring constant; and

-   -   d₀ is the initial value of the gap distance 78.        As discussed in the Background Section of this application, the        range in which the forces of Equations I and II are in stable        equilibrium occurs when the value (d₀−d) is between zero and        d₀/3. At (d₀−d)>d₀/3, the electrostatic force of attraction of        equation (1) overcomes the spring force of Equation II, such        that the reflector 74 snaps to reflector 72, which is        undesirable. This occurs because when the reflector 74 is beyond        the d₀/3 position, excess charge is drawn onto reflectors 72 and        74 due to increased capacitance, which in turn increases the        attractive force of Equation I between reflectors 72 and 74,        causing reflector 74 to pull towards reflector 72.

However, the force between reflectors 72 and 74 of equation I caninstead be written as a function of charge according to the followingEquation III: Equation  III: ${F = \frac{- Q^{2}}{2\quad ɛ\quad A}},$

where Q is the charge on the capacitor. With force F as a function ofcharge Q rather than distance d, it can be seen that the position ofreflector 72 can be effectively controlled over the entire gap distanceby controlling the amount of charge on reflectors 72 and 74.

Furthermore, micro-electromechanical device 70 has a mechanical timeconstant that causes delays in the movement of reflector 72 resultingfrom changes in charge Q on the variable capacitor. The mechanical timeconstant can be controlled by, among other things, the material used inspring mechanism 82 and by an environment in whichmicro-electromechanical device operates. For example, the mechanicaltime constant of micro-electromechanical device 70 will have one valuewhen operating in an environment comprising air and another value whenoperating in an environment comprising helium.

Charge control circuit 32 utilizes each of these characteristics tocontrol the gap distance over substantially the entire range. Byapplying a selectable control voltage to micro-electromechanical device70 based on a duration of an enable signal, wherein the duration is lessthan device 70's mechanical time constant, the variable capacitance ofdevice 70 appears to be “fixed” for the duration that the referencevoltage is applied. As a result, the desired charge (Q) accumulated onthe reflectors 72 and 74 from application of the selected referencevoltage can be determined by Equation IV below:

Equation IV:Q=C _(INT) *V _(REF),

-   -   where V_(REF) is the selected reference voltage; and    -   C_(INT) is the initial capacitance of micro-electromechanical        device 70.        By keeping the duration of the enable signal (i.e., the        electrical time constant) less than the mechanical time        constant, the reference voltage is applied to        micro-electromechanical device 70 for a specific duration to        deliver the desired charge and then removed. Once the reference        voltage has been removed, micro-electromechanical device 70 is        floating, or tri-stated, thus preventing additional charge from        accumulating and enabling effective control of the gap distance        for an increased control range relative to direct voltage        control of micro-electromechanical device 70.

Although the description of the preceding paragraphs is with respect toan ideal parallel-plate capacitor and an ideal linear spring restoringforce, those of ordinary skill within the art can appreciate that theprinciple described can be adapted to other micro-electromechanicaldevices 70, such as interference-based or diffraction-based displaydevices, parallel plate actuators, non-linear springs and other types ofcapacitors. With display devices, when the usable range is increased,more colors, saturation levels, and intensities can be achieved.

In one embodiment, micro-electromechanical device 70 is a parallel plateactuator 70. Parallel plate actuator 70 includes a flexure 80 in aspring mechanism 82. Spring mechanism 82 is adapted to support a firstplate 72 and provide a restoring force to separate the first plate 72from the second plate 74. Flexure 80 is attached to spring mechanism 82and is adapted to support second plate 74. The spring mechanism 82 andflexure 80 maintain the first plate 72 in an approximately parallelorientation with respect to the second plate 74 at a deflection distance78 or gap distance 78.

In one embodiment, micro-electromechanical device 70 is a passive pixelmechanism 70. The pixel mechanism 70 includes an electrostaticallyadjustable top reflector 72 and bottom reflector 74 which are configuredto define a resonant optical cavity 76. Charge control circuit 32 isconfigured to select a visible wavelength of the passive pixel mechanism70 by storing a charge having a desired magnitude on top reflector 72and bottom reflector 74, to thereby control a gap distance 78.

FIG. 3 illustrates schematically at 90 one embodiment of switch circuit40 according to the present invention. Charge control circuit 32includes a first switch 91 and a second switch 93. In one embodiment,first switch 91 is a −p-channel metal-oxide-semiconductor (PMOS) devicehaving a gate 94, a source 96, and a drain 98. In one embodiment, secondswitch 93 is an n-channel metal-oxide-semiconductor (NMOS) device havinga gate 104, a drain 106, and a source 108.

First switch 91 receives the selected reference voltage (V_(REF)) atsource 96 via path 54 and the enable signal at gate 94 via path 56.Drain 98 is coupled to first conductive plate 42 ofmicro-electromechanical device 34 via path 60. Second switch 93 iscoupled across micro-electromechanical device 34 with drain 106 coupledto first conductive plate 42 and source 108 coupled to second conductiveplate 44 via ground. Second switch 93 receives the clear signal at gate104 via path 58.

Switch circuit 40 operates as described below to cause a charge having adesired magnitude to be stored on first and second conductive plates 42and 44. Initially, the enable signal is at a “high” level, the clearsignal is at a “low” level, and the reference voltage is at a selectedvoltage level. The clear signal is then changed from a “low” level to a“high” level, causing second switch 93 to turn on and take firstconductive plate 42 to ground, thereby removing any charge that may havebeen stored on variable capacitor 46. The signal is then returned to the“low” level causing second switch 93 to again turn off.

The enable signal is then changed from the “high” level to a “low”level, causing first switch 91 to turn on to thereby apply the referencevoltage to variable capacitor 46 and cause a desired charge toaccumulate on first and second conductive plates 42 and 44, and therebyset the gap distance 48 to a desired distance. The enable signal stays“low” for a predetermined duration before returning to the “high” levelcausing first switch 91 to again turn off, decoupling the referencevoltage from micro-electromechanical device 34. At this point, themicro-electromechanical device is tri-stated, or isolated, and chargecan no longer flow. The predetermined duration is shorter than amechanical time constant of micro-electromechanical device 34 resultingin the variable capacitor 46 appears to be substantially “fixed” duringthe predetermined duration so that the stored charge can be calculatedusing Equation IV. Thus, in one embodiment, the predetermined durationis a fixed value and the value of the reference voltage is varied tothereby control the magnitude of the charge stored on variable capacitor46

In one embodiment, switch circuit 40 does not include second switch 93and does not receive the clear signal to first remove any stored chargefrom variable capacitor 46 prior to charging variable capacitor 46 to adesired magnitude. Thus, rather than charging variable capacitor 46 froma value of zero each time the variable gap distance is changed, thereference voltage is modified as required to transition from one gapdistance to another gap distance. To transition to a smaller gapdistance from a large gap distance, the reference voltage is increasedto add charge to variable capacitor 46. To transition to a larger gapdistance from a smaller gap distance, the reference voltage is decreasedto thereby remove charge from variable capacitor 46.

FIG. 4 is a block diagram illustrating an exemplary embodiment of amicro-electromechanical system 120 according to the present invention.Micro-electromechanical system 120 comprises an M-row by N-column arrayof micro-electromechanical (MEM) cells 122, with each cell 122comprising a micro-electromechanical device 34 and switch circuit 40.Although not illustrated for simplicity, each mirco-electromechanicaldevice 34 further comprises a first conductive plate 42 and a secondconductive plate 44 forming a variable capacitor 46 separated by avariable gap distance 48

Each switch circuit 40 is configured to control the magnitude of astored charge on variable capacitor 46 of its associatedmicro-electromechanical device 34 to thereby control the associatedvariable gap distance 48. Each row of the M rows of the array receives aseparate clear signal 124 and enable signal 126, for a total of M clearsignals and M enable signals, with all switch circuits 40 of a given rowreceive the same clear and enable signals. Each column of the N columnsof the array receives a separate reference voltage (V_(REF)) 128, for atotal of N reference voltage signals.

To store, or “write”, a desired charge to each micro-electromechanicaldevice 32 of a given row of micro-electromechanical cells 122, areference voltage having a selected value is provided to each of the Ncolumns, with each of the N reference voltage signals potentially havinga different selected value. The clear signal for the given row is then“pulsed” to cause each of the switch circuits 40 of the given row toremove, or clear, any potential stored charge from its associatedmicro-electro mechanical device 34. The enable signal for the given rowis then “pulsed” to cause each switch circuit 40 of the given row toapply its associated reference voltage to its associatedmicro-electromechanical for the predetermined duration. As a result, astored charge having a desired magnitude based on the value of theapplied reference voltage is stored on the associated variable capacitorto thereby set the variable gap distance based on the desired magnitudeof the stored charge. This procedure is repeated for each row of thearray to “write” a desired charge to each micro-electromechanical cellof the array.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. Those with skill in the chemical,mechanical, electro-mechanical, electrical, and computer arts willreadily appreciate that the present invention may be implemented in avery wide variety of embodiments. This application is intended to coverany adaptations or variations of the preferred embodiments discussedherein. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A charge control circuit for controlling a micro-electromechanicalsystem (MEMS) device having a variable capacitor formed by a movablefirst conductive plate and a fixed second conductive plate separated bya variable gap distance, the charge control circuit comprising: a switchcircuit configured to receive a reference voltage having a selectedvoltage level and configured to respond to an enable signal having aduration at least as long as an electrical time constant of the MEMSdevice, but shorter than a mechanical time constant of the MEMS device,to apply the selected voltage level across the first and second platesfor the duration to thereby cause a stored charge having a desiredmagnitude to accumulate on the variable capacitor, wherein the variablegap distance is a function of the magnitude of the stored charge.
 2. Thecharge control circuit of claim 1, wherein the switch circuit comprises:a first switch coupled to the micro-electromechanical device andconfigured to provide the reference voltage to themicro-electromechanical device for the duration of, and in response to,the enable signal.
 3. The charge control circuit of claim 2, wherein thefirst switch comprises: a p-channel metal-oxide-semiconductor (PMOS)device having a source configured to receive the reference voltage, agate configured to receive the enable signal, and a drain configured toprovide the reference voltage to the micro-electromechanical devicebased on the enable signal.
 4. The charge control circuit of claim 1,wherein the charge control circuit further comprises: a variable powersupply configured to provide the reference voltage at the selectedvoltage level; and a controller configured to provide the enable signaland to control the selected voltage level of the reference voltageprovided by the variable power supply.
 5. The charge control circuit ofclaim 1, wherein the switch circuit further comprises: a second switchcoupled across the first and second conductive plates and configured todischarge the stored charge from the variable capacitor in response to aclear signal.
 6. The charge control circuit of claim 5, wherein thesecond switch comprises: an n-channel metal-oxide-semiconductor (NMOS)device having a gate configured to receive the clear signal and a drainand source coupled across the first and second conductive plates.
 7. Thecharge control circuit of claim 5, wherein the charge control circuitfurther comprises: a variable power supply configured to provide thereference voltage at the selected voltage level; and a controllerconfigured to provide the enable signal and the clear signal, and tocontrol the selected voltage level of the reference voltage provided bythe variable power supply.
 8. A micro-electromechanical cell comprising:a micro-electromechanical system (MEMS) device having a variablecapacitor formed by a movable first conductive plate and a fixed secondconductive plate separated by a variable gap distance; and a switchcircuit configured to receive a reference voltage having a selectedvoltage level and configured to respond to an enable signal having aduration at least as long as an electrical time constant of the MEMSdevice, but shorter than a mechanical time constant of the MEMS device,to apply the selected voltage level across the first and second platesfor the duration to thereby cause a stored charge having a desiredmagnitude to accumulate on the variable capacitor, wherein the variablegap distance is a function of the magnitude of the stored charge.
 9. Themicro-electromechanical cell of claim 8, wherein the switch circuitcomprises: a first switch coupled to the micro-electromechanical deviceand configured to provide the reference voltage to themicro-electromechanical device for the duration of, and in response to,the enable signal.
 10. The micro-electromechanical cell of claim 9,wherein the first switch comprises: a p-channelmetal-oxide-semiconductor (PMOS) device having a source configured toreceive the reference voltage, a gate configured to receive the enablesignal, and a drain configured to provide the reference voltage to themicro-electromechanical device based on the enable signal.
 11. Themicro-electromechanical cell of claim 8, wherein the switch circuitfurther comprises: a second switch coupled across the first and secondconductive plates and configured to discharge the stored charge from thevariable capacitor in response to a clear signal.
 12. Themicro-electromechanical cell of claim 11, wherein the second switchcomprises: an n-channel metal-oxide-semiconductor (NMOS) device having agate configured to receive the clear signal and a drain and sourcecoupled across the first and second conductive plates. 13-32.(Cancelled).